PHILADELPHIA—In December, Spark Therapeutics and Pfizer Inc. announced updated
preliminary data from the first nine infused participants in the ongoing Phase 1/2 clinical trial of investigational SPK-9001 for hemophilia B.

Hemophilia B is a rare, genetic bleeding disorder, most commonly affecting males, that causes the blood to take longer to
clot than normal. This can cause excessive and recurrent bleeding, often into the muscles and joints, and carries the potential to be life-threatening.
Patients with hemophilia B have a deficiency of a protein in the blood, clotting factor IX, which is currently treated with intravenous infusions of either
plasma-derived or recombinant factor IX to control and prevent bleeding episodes.

As of Nov. 30, 2016, all nine
infused patients experienced a reduction of the total clotting factor IX concentrates consumption by 1.13 million international units when compared to their
factor IX concentrates usage in the year before vector administration. Seven of those nine experienced consistent and sustained factor IX activity levels,
with a mean steady-state level greater than 28 percent. So far, no participants have developed factor IX inhibitors or experienced thrombotic events after
the vector administration, and no other serious adverse effects have been reported after SPK-9001 administration.

The first patient to reach the one-year earmark has reduced his number of intravenous factor IX infusions to zero without having any bleeds, and his
steady-state factor IX activity level was reported at 33 percent of normal. By comparison, in the year prior to participation in the trial, he infused factor
IX concentrates as a precaution a total of 98 times and still experienced four traumatic bleeds.

To date, eight of
the patients have required no factor IX concentrates to prevent or control bleeding events since the day after vector administration, though one participant
with severe joint disease self-administered two precautionary infusions on separate occasions for suspected bleeds.

Two of the nine participants experienced asymptomatic, transient elevation in liver enzymes associated with an immune response to the Spark100 vector
capsid during the first four to eight weeks following administration. In one case, this response was accompanied by a decline in factor IX activity level
from 32 to 12 percent. With the other patient, steroids were administered more promptly, experiencing a decline from a peak factor IX activity level of 71 to
68 percent. Both participants were put on a tapering course of corticosteroids, and, to date, neither participant has experienced any bleeds nor has required
infusion of replacement factor IX concentrates.

“We will monitor these two participants carefully as we
continue to taper the corticosteroids,” said Dr. Katherine A. High, president and chief scientific officer at Spark Therapeutics. “It’s
important to remember that the immune response to the capsid typically is transient, and in both cases, seems to have been arrested by corticosteroids. Once
corticosteroids have been discontinued altogether, levels of expression will be the best measure of the efficacy of this approach. The experience we have
gained in immuno-monitoring and in clinical management of the immune response in the hemophilia B trial will further inform our upcoming hemophilia
studies.”

Thus far, six of the nine patients have reported increased physical activity and improved quality
of life based on the Haemophilia Quality of Life Questionnaire for Adults, a validated instrument that measures health-related quality of life in adults with
hemophilia. Two of the remaining participants received the gene therapy product too recently to evaluate quality-of-life measures.

Spark Therapeutics and Pfizer entered into a collaboration in 2014 for the SPK-FIX program, including SPK-9001, but
Spark’s founding team carries history of hemophilia gene therapy research and clinical development that goes back almost 30 years.

SPK-9001, a novel investigational bio-engineered adeno-associated virus capsid expressing a codon-optimized, high-activity
human factor IX variant enabling endogenous production of factor IX, has received Breakthrough Therapy status and Orphan Drug designation from the U.S. Food
and Drug Administration. Per the terms of the collaboration, Spark is responsible for conducting all Phase 1/2 studies for any product candidates, while
Pfizer assumes responsibility for pivotal studies, any regulatory activities and potential global commercialization of any products that may result from the
collaboration.

“Giving people living with hemophilia B treatment options that potentially minimize or
eliminate the need for infusions of factor concentrates is one of the main goals of conducting clinical studies of investigational gene therapies,”
said High. “While continued observation and larger cohorts are needed, these updated preliminary data continue to be encouraging and suggest the
potential of investigational SPK-9001 to deliver a consistent, sustained and therapeutically meaningful level of factor IX activity through a one-time
intravenous administration.”